Brucella ovis is a species of gram negative Cocobacillius from the Brucellacae family1, which causes infections in various animal species, some of which are zoonotic (B.abortus, B.melitensis, B.suis). Brucella ovis is non-zoonotic infectious disease that is transmitted from ram to ram, or a ram to ewe(s). Brucella ovis has a worldwide distribution with known occurrences in Australia, New Zealand, Africa, South East Asia, South America and several European countries 1.
The disease can cause clinical as well as chronic sub-clinical disease in rams afflicted by the presence of Brucella ovis. The disease manifests itself by testicular alterations in individual rams, ram infertility and the rare occurrence of abortions in ewes.
Whilst the ewe is not regarded as a major source for the infection, she still plays a role in the epidemiology of the infection. Infection of ewes has been reported to result in late-term abortions or birth of weak lambs which die soon after birth2. Ewes are a recognized vector for mechanical transmission of B.ovis between rams by harbouring the organism in the vagina3. Transmission between rams may occur when a ewe mates an infected ram and is then mated by an uninfected ram.
British breeds have been shown to be more susceptible to the disease than merinos and rams have shown to be the more clinically affected as compared to ewes. In most cases, infected B.ovis properties undergo an eradication program based exclusively on the serological testing and culling of seropositive rams.
A producer around the Young area grew concerned as he was noticing several aborted fetuses about his lambing paddock. This producer had been vaccinating for Campylobacter for the last 4 years and was up to date on all of his management practices. The mob of ewes in question were 3-4 year old ewes.
A subsequent property visit followed where 2 aborted, near to full term foetuses were taken from a ewe that was potentially not vaccinated against Camplyobacter. Samples were sent to EMAI for analysis and a growth of Campylobacter was obtained. With the assumption that this mob might undergo further abortions (due to lack of vaccination against Camplylobacter spp), this was put down as the cause for the abortions to date.
A week later Riverina LLS vets were called out to investigate abortions in 2 other mobs. A fresh foetus was taken for analysis at EMAI. Possible causes for late-term abortions in this case were: Campylobacter, Toxoplasmosis, Ovine Brucellosis (OB), listeriosis and Chlamydia Psittaci (Enzootic abortion). Samples were checked against each of the above diseases and a diagnosis of Brucellosis was confirmed via culture and PCR.
A thorough examination of the rams was conducted, with approximately 60% found to have thickened epididymides particularly around the tail of the epididymis. Several rams had grossly enlarged, pendulous and firm testicle or testicles A further 25% had titres of up to 256 (doubling of serum dilutions, with 8 being the lowest and 256 being the highest) after Complement Fixation testing was conducted. All of these rams were culled immediately from the flock. Re-testing of the rams occurred at 4-6 week intervals, with the remaining rams being separated into two smaller mobs to aid in the eradication and salvage program. Unfortunately the titres from each of these mobs kept returning positive titres for OB. As such, the owner opted to sell all of the remaining rams direct to slaughter and subsequently purchased a new stock of rams prior to his next joining season.
A producer from the Nyngan area contacted the District Veterinarian after she noticed an aborted fetus in the sheep yards as they were being brought in for scanning. The owner had 90% of ewes in lamb at the time of that scanning. The owner reported that the source of the rams was from a producer who had previously been diagnosed with Ovine Brucellosis. Differential diagnoses for this case included Listeriosis, Camplylobacter, Pestivirus, Chlamydia and B.ovis. Subsequent testing on the foetus by pathologists at EMAI revealed it to be PCR positive to B.ovis and negative for the other suspects.
B.ovis is known as a rare cause of abortion marked by severe placentitis4. It is also a known cause of endometritis in ewes. Brucella abortions occur late in gestation, resulting in placentitis with edema and necrosis of the cotyledons and thickened, leathery intercotyledonary areas4. Many fetuses aborted due to B ovis are alive at the beginning of parturition, although fetuses can be mummified or autolyzed. Histopathological lesions from infected fetuses have been found in the liver, lungs, intestines, kidney, abomasum, lymph nodes and placenta4, 5.
B.ovis can be excreted in milk and this has been demonstrated naturally and in experimental trials. Grillo et al. 5 have shown that mammary infections can persist in ewes for up to 2 successive lactations post infection. Grillo has also hypothesized the theory of a latent infection that persists with lambs that suckle from infected B.ovis ewes. A heavy and persistent B.ovis mammary infection has the potential of being passed on to a suckling lamb – or at least that’s what they hypothesize and believe is worthy of epidemiological consideration.
Questions often asked by producers that are diagnosed with OB and are concerned with ewes on their property:
When does the ewe abort when infected with B.ovis?
How long does the ewe stay infected with B.ovis?
Do lambs that are born to what could be infected ewes demonstrate immunity?
Can the ewe pick up the organism through means other than via a ram mounting her?
Although B.ovis is a disease most commonly affected sheep, it also affects goats, Mouflon, white-tailed deer and red-tailed deer. A 2011 study by Ridler et al.10 showed that natural infection of hinds to B.ovis is unlikely to cause abortions. Similar to the studies undertaken by Osburn and Kennedy4, experimental infection may result in abortion and perinatal death.
Ewe to ram transmission of the disease is reported to be a rare occurrence. However, B.ovis can be actively excreted vaginally at the time of weaning by a significant proportion of infected ewes5, so the possibility of direct transmission during mating should not be excluded when undertaking abortion investigations in sheep.
Trials done over the decades have shown that although infection with B.ovis induces a severe endometritis in affected ewes, B.ovis has a relatively low capacity to induce abortion in sheep.
It is worth noting that EMAI prefers that submitters send entire chilled (not frozen) fetuses to them as this increases the likelihood of culturing the organism and/or getting a B.ovis PCR result. If this for some reason is unable to be done, Osburn and Kennedy identified that cultures from the lung (pneumonic lesions present) and abomasal contents should be taken, as these were the most likely sites for the growth of the organism to occur.